It's been a couple years since I last posted about one of the biggest things that revolutionized my shooting -- back button focus. And I still talk to photographers who aren't familiar with it and haven't taken the steps to adopt this superior focusing method. If you're one of them -- STOP EVERYTHING NOW. And do this. You'll thank me :).
In my conversations with other photographers many of them share frustration with not being able to focus as consistently or accurately as they'd like. The biggest thing that has impacted my images in the realm of focus is switching from using the shutter button to control focus to using the back * button to control focus.
The idea is to separate the focus function from the shutter function. I love it because when I'm shooting a stationary portrait or some situation where the camera to subject distance is constant, I can lock in the focus (push the * button with the focal point on the spot I want in focus, then let go of the button) and shoot away, even re-composing, and the focus will stay dead-on. This has also freed me up from constantly changing my focus points which I used to do because I wasn't confident in the shutter locking the focus. I now leave the center focus point selected and recompose after locking in the focus using the * button on the back of the camera.
In the situation of a moving subject, I also like the back button because it allows me to track the subject and still get consistent focus. For processionals and recessionals I put my center focus point on a point of high contrast -- generally on the man's torso at the point where his jacket meets his shirt:
The reason I put it there rather than on an eye is that, with moving subjects, it can be difficult to keep the focus point on a small area. If I focused on his eye, there's a good chance the focus box will overlap his head and the background and I could end up with plenty of beautiful shots with the background pristinely focused and my subject blurry. So it's better to pick a spot like a torso that has more surface area. Also, my camera is generally equidistant from the torso and the eye (they're on the same plane equidistant from the camera) so even if I'm shooting with a wide open aperture (the above shot was taken at f1.2) I will get the eye in focus. With my camera in Al Servo mode (always leave it there -- never take it off) I can just hold the * button down and keep shooting constantly and the focus will track the subject. It's a beautiful thing!
If you want to try back-button focusing, you need to change some of your custom functions. Follow these instructions once and you're set! You should notice that your camera will focus when pushing the * button but when you push the shutter button, your focus will not be impacted.